This does not look right. I think that the camera was probably not manually focused at 75 Inches. Please re-perform test, making sure to manually focus as accurately as possible on the back portion of chair. Provide Distance, Focal Length, F-Number, and shot of the DOF Indicator scale (as before).

Same subject distance 75 inches. f = 6.9 mm, F/1.6. Manually focussed (and verified that a pre-AF also brings the DOF slider to the same point as I had done manually).